Page 292 - Water and wastewater engineering

LIME–SODA SOFTENING 7-9

4. Removal of noncarbonate hardness due to calcium.
To remove noncarbonate hardness due to calcium, additional carbonate in the form of
soda ash must be added
2
Ca Na CO 3 CaCO (s) 2Na (7-16)
2
3
5. Removal of noncarbonate hardness due to magnesium .
To remove noncarbonate hardness due to magnesium, both lime and soda must be
added. The lime provides the hydroxyl ion for precipitation of the magnesium.
2
Mg Ca(OH) 2 Mg OH 2 ) ( s ) Ca 2 (7-17)
(
Although the magnesium is removed, there is no change in the hardness because the
calcium is still in solution. To remove the calcium, soda ash must be added.

2
2
Ca Na CO 3 CaCO s() Na (7-18)
3
2
Note that this is the same reaction as the one to remove noncarbonate hardness due to
calcium.
These reactions are summarized in Figure 7-5 . Although the reactions shown above use lime
and soda ash as sources of hydroxyl ion and carbonate ion, other sources may be used. For ex-
ample, sodium hydroxide (NaOH) can be substituted for calcium hydroxide.

pH. Solubility relationships are generally more complex than implied by the discussion to this
point. In addition to the solubility product, other equilibria affect the concentration of the ions
present. Other ions may form salts with less solubility than the ones assumed to result from the
solubility equilibrium. Reactions of the cation or anion with water to form hydroxide complexes or

Neutralization of carbonic acid
CO Ca(OH) CaCO (s) H O
2 2 3 2
Precipitation of carbonate hardness
–
Ca 2 2HCO Ca(OH) 2CaCO (s) 2H O
3 2 3 2
–
Mg 2 2HCO Ca(OH) MgCO CaCO (s) 2H O
2
3
3
2
3
MgCO Ca(OH) 2 Mg(OH) (s) CaCO (s)
2
3
3
Precipitation of noncarbonate hardness due to calcium
Ca 2 Na CO 3 CaCO (s) 2Na
3
2
Precipitation of noncarbonate hardness due to magnesium
Mg 2 Ca(OH) Mg(OH) (s) Ca 2
2 2
Ca 2 Na CO 3 CaCO (s) 2Na
3
2
FIGURE 7-5
Summary of softening reactions. (Note: The chemical added is
printed in bold type. The precipitate is designated by (s). The
arrow indicates where a compound formed in one reaction is used
in another reaction.) (Source: David and Cornwell, 2008.)